1. Technical Field
The invention is related to methods for forming ceramic matrix composites having ceramic fibers coated with a toughening layer which facilitates fiber de-bonding and pull-out in the wake of a crack in the matrix.
2. Background Art
Fiber reinforced ceramic matrix composites comprise a weave of ceramic fibers embedded in a ceramic matrix. One way of improving the mechanical properties of such a composition is to provide a coating over the fibers which is stable and resistant to oxidation and which promotes fiber de-bonding at the tip of an advancing crack and fiber bridging and eventually pull-out in the wake of an advancing crack in the composite. This feature enhances the toughness, strength and strain to failure of the composite because the fibers remain mostly immune to advancing cracks in the matrix. The history of development of this technique is described in U.S. Pat. No. 4,885,199 to Corbin et al.
For certain applications, the ceramic matrix composite must be stable at temperatures above 2200 degrees F. in an oxidizing environment. Well-known fiber coatings such as carbon and boron nitride are not stable under such conditions. Any material to be substituted for a fiber coating in place of the carbon or boron nitride must be both resistant to oxidation and must possess sufficient strength to transfer loads from the matrix to the fiber while having a low shear strength to promote debonding between the fiber and the matrix in the presence of an advancing crack. Furthermore, the coating must be easy to apply to macrofibers, fiber tows and to a weave of ceramic fibers during manufacturing.
U.S. Pat. No. 4,946,807 to Okuno et al. discloses a SiC whisker composite in which zirconia is added as a particle, not as a coating on continuous fibers. U.S. Pat. No. 4,657,877 to Beecher et al. discloses SiC whiskers in mullite or alumina toughened by adding zirconia particles, in which the zirconia is not a fiber coating. U.S. Pat. No. 4,749,667 to Jun et al. discloses SiC whiskers in a composite with alumina containing particulate zirconia, in which the zirconia is not a coating on a continuous fiber. U.S. Pat. No. 4,920,838 to Brandt et al. discloses a ceramic cutting tool in which zirconia is present in particulate form, not as a coating on a continuous fiber. U.S. Pat. No. 4,774,209 to Gadkaree et al. discloses whisker reinforced zirconia-mullite ceramics in which zirconia is present as dispersed particles and not a coating on a continuous fiber. U.S. Pat. No. 4,218,253 to Dworak et al. discloses improving ductility of sintered alumina by including particulate zirconia, and has nothing to do with using zirconia as a fiber coating. U.S. Pat. No. 4,298,385 to Claussen et al. discloses zirconia present in particulate form in a sintered ceramic matrix such as alumina to cause fractures in the matrix, and has nothing to do with using zirconia as a coating on continuous fibers or a fiber reinforced ceramic matrix. Japanese Patent No. 54-70333 discloses surface coating for inorganic fibers to produce cement products using added zirconia.
The prior art as described in U.S. Pat. No. 4,885,199 referenced above typically relied upon the characteristics of the inherently weak shear strength of carbon and boron nitride coatings to achieve desired characteristics, such as toughening. For applications in the high temperature oxidizing environments described above, the intrinsic properties of the coating composition would have to provide all of the necessary features, including fiber de-bonding and pull-out as well as imperviousness to oxidation and high temperatures. The problem with this approach is that it is very difficult to select the best fiber coating material for a given ceramic fiber sc as to optimize all of the foregoing features in the same coating material composition.
Thus, one object of the present invention is to depart from the prior art approach of finding a coating composition which provides all of the necessary features, and instead find a mechanical approach in which all, or at least some, of the desired features (such as fiber de-bonding and pull-out in the wake of an advancing crack in the matrix) are realized through the mechanical features of the coating and coating/fiber interface, as distinguished from the inherent features of the composition. Such a mechanical approach has many advantages and, in most cases, allows greater choice in selecting the materials for use in the coating. For example, the materials can be chosen to meet only the requirement of resistance to oxidation and stability at high temperatures, while the remaining requirements (e.g., fiber de-bonding and pull-out) are met by mechanical features in the coating or coating/fiber interface. In fact, the same composition as the fiber and/or the matrix would be a candidate for the coating. This advantage will become clear in the description of the invention which follows the conclusion of this description of the background art.
In the present invention, the mechanical feature which promotes the requisite tendencies (e.g., fiber de-bonding and pull-out in the wake of an advancing crack in the fiber/matrix composite) is the presence of pre-formed cracks in the fiber coating. A method of forming microcracks in a ceramic matrix is disclosed in the background discussion in U.S. Pat. No. 4,732,877 to Olson et al., which teaches the addition of zirconia to the ceramic matrix and heating the zirconia through its tetragonal phase change temperature to form microcracks in the ceramic matix. Such microcracks enhance the strength of the ceramic matrix by diffusing advancing cracks. One problem is that characteristics of the material produced by this method, such as the thermal expansion coefficient of the fiber coating, are not readily controlled or varied, being determined by the monoclinic-tetragonal phase change of the zirconia. Moreover, the referenced patent to Olson et al. has nothing to do with forming microcracks in a thin coating surrounding ceramic fibers immersed in a ceramic matrix. Instead, Olson et al. teach a method for maximizing the density of a zirconia diffusion barrier coating formed on the ceramic fibers prior to their immersion at very high temperatures in a ceramic matrix. There appear to be no known methods for forming a pre-cracked coating on a ceramic fiber in a ceramic matrix.
Accordingly, it is an object of the invention to provide methods ideally suited for forming a pre-cracked coating on a ceramic fiber in a ceramic matrix.
It is a related object of the invention to provide a method of forming a pre-cracked metal oxide or non-oxide coating around a ceramic fiber in which the oxide or non-oxide is formed or transformed in situ after the fiber has been embedded in a ceramic matrix.
It is another object of the invention to form a pre-cracked coating on a ceramic fiber in a ceramic matrix whose properties are selectively determined during processing.